Spatially resolved STIS spectroscopy of α Orionis:: evidence for nonradial chromospheric oscillation from detailed modeling

Four spatially resolved near-UV raster scans across the chromospheric disk of a Ori, obtained with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, reveal mean chromospheric infall from 1998 January to April, which reversed to upflow in deeper layers between 1998 September and 1999 March. In 1998 September we detect systematic reversals in the component maxima of four double-peaked emission lines of Si I (UV 1), Fe II (UV 36), Fe II (UV 61), and Al II] (UV 1), when scanning across the UV disk. Detailed modeling of the Si I lambda 2516 resonance line with radiative transport calculations in spherical geometry constrain the mean radial velocity structure in the projected slit area (25x100 mas) for different aperture positions, observed off-limb to 157.5 mas. Hence, we determine with semiempirical models that these spatial reversals of emission-line components correspond to average opposite flow velocities of similar to2 km s(-1) across the chromospheric disk. We determine that the chromospheric velocity field cannot be represented by a unique radial velocity structure across the stellar disk in order to match the observed peak ratios of this raster scan. These subsonic velocities indicate (local) nonradial movements of chromospheric fluid in confined regions during a chromospheric oscillation phase, which reverses from global contraction into expansion over this monitoring period of 15 months.